New manufacturing processes and new applications are forcing power supplies to lower voltages (3.3 v now, with 2.4 v and 1.5 v being expected soon). Advanced Phase-Locked Loops require stable oscillators which may be varied in frequency by a control signal.
To help achieve frequency stability, oscillators integrated into a noisy VLSI environment often use a regulator to generate a quiet power supply. This usually has to be at an even lower voltage than the normal power supply.
There is thus a desire to provide oscillators which can work at these very low supply voltages and still produce high quality, high frequency output signals.
Reference is made to IBM Technical Disclosure Bulletin, Vol. 31, No. 2, July 1988, pages 154 to 156 "CMOS Ring Oscillator with controlled frequency" which describes a ring oscillator using CMOS transistors and is designed to give an almost sinusoidal output. This design suffers from stability problems outside a narrow range of frequencies. In particular, as the frequency increases, the amplitude decreases and it becomes difficult to convert the signal to CMOS levels.